Conductive materials have magnetostrictive properties that cause the materials to change shape in the presence of an applied magnetic field. The inverse is also true. When a force is applied to a conductive material, the magnetic properties, such as magnetic permeability, of the material change. A magnetostrictive sensor may sense the changes in magnetic permeability and, because the changes are proportional to the amount of stresses applied to the conductive material, the resulting measurement may be used to calculate the amount of stress.
Stationary magnetostrictive sensors proximate to a moving conductive material, such as a rotating shaft, sense the magnetic permeability of an air gap defined between the magnetostrictive sensor and the conductive material (e.g., mechanical vibration) and variation in the conductive material properties (e.g., runout) as well as the permeability of the conductive target. The changes in the magnetic permeability as a result of stress being applied to the conductive material, however, may be small compared to the mechanical vibration and runout of the conductive material, making accurate measurement of stress in the conductive material difficult. In some instances, the runout and/or mechanical vibration result in signal noise that may have amplitudes greater than that of the stress signal, thereby completely obscuring the stress signal.